Salt Lake City Could See Bigger Earthquakes

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Two faults bounding Utah's biggest city may combine to produce
especially powerful earthquakes, geologists will report in Salt
Lake City today (April 17) at the annual meeting of the
Seismological Society of America.

Utah's biggest earthquake
fault runs east of Salt Lake City, at the base of the steep
Wasatch Mountains. About 75 percent of the state's population
lives near the 240-mile-long (385 kilometers) Wasatch Fault,
according to the Utah Geological Survey. Its last big earthquake
hit in 1600, 247 years before Mormon settlers arrived.

To the west, in urban
Salt Lake City, a 4-mile-wide (6 km) zone of fault segments
called the West Valley Fault Zone stretches north-northwest for 9
miles (14 km) beneath the valley.

Trenches along a portion of the West Valley fault zone, near Salt
Lake City's airport, reveal that both the West Valley and Wasatch
faults seem to rupture simultaneously during earthquakes,
scientists will report today at the meeting.

While dating techniques can't confirm that the earthquakes were
synchronous, instead of within a few days, month or years,
modeling suggests they strike at the same time, said Christopher
DuRoss, study co-author and a geologist at the Utah Geological
Survey.

"Based on models of how the crust would behave, we expect the
West Valley Fault Zone would rupture instantaneously with the
Salt Lake City segment," DuRoss told OurAmazingPlanet.

If both fault zones ruptured during an earthquake, it would mean
more shaking for Salt Lake City, which sits atop soft lake
sediments, the kind that experience liquefaction during severe
earthquakes. In the 2011 Christchurch, New Zealand earthquake,
liquefaction destroyed the city's downtown core. In Salt Lake
City, planners are also concerned about the risk of flooding from
waves in the
Great Salt Lake and landslides in mountain canyons during a
major earthquake.

Residents of Salt Lake will get a better picture of their risk
when the Utah Geological Survey and U.S. Geological Survey
release updated hazard maps in 2014, which are based on today's
presentation and other recent work, DuRoss said. [ What's
the Most Earthquake-prone State in the U.S.? ]

The Wasatch Fault is divided into 10 segments, which act mostly
independently, researchers think. The 25-mile-long (40 km) Salt
Lake City segment is thought to be one of the most hazardous,
with the probability of a large quake (magnitude 7.0) put at 16.5
percent in the next 100 years, according to the Utah Geological
Survey. However, that earthquake forecast is now out-of-date,
thanks to new research, and will be updated next year by the
Working Group on Utah Earthquake Probabilities, DuRoss said.

Trenches find big quakes

DuRoss and study co-author Michael Hylland looked at the link
between the Wasatch Fault and the West Valley Fault Zone with
trenches dug near the Salt Lake City airport, where the shrinking
Great Salt Lake has exposed West Valley fault traces. For the
Wasatch fault, the team dug new trenches near the University of
Utah.

Disturbed sediment layers indicate four
big earthquakes on the West Valley fault broke ground in the
past — 15,700, 13,000, 12,300 and 5,500 years ago, said Hylland,
also a geologist at the Utah Geological Survey.
Radiocarbon and optical luminescence dating ties the broken
ground to earthquake records in trenches along the Salt Lake City
section of the Wasatch Fault.

More complete sediment records exist for the Salt Lake City
section of the Wasatch Fault, with nine prehistoric temblors
found, Hylland said.The last big earthquake on the Salt Lake City
segment was 1,400 years ago. The quakes hit every 1,300 to 1,500
years, researchers think.

"From what we can see, it looks like the frequency is about the
same" on the two fault zones, Hylland told OurAmazingPlanet.
"What it really comes down to is 'how active is the Salt Lake
City segment?'" Hylland said. "That's the real driver of the
hazard for Salt Lake Valley."

The separate faults likely merge into a single fault deep beneath
the valley, Hylland said. The West Valley faults angles to the
east, and the Wasatch Fault dips to the west.

Movement on both faults is up-down. They are both normal faults,
sliding one block of the Earth's crust away from another block
during an earthquake.

Editor's note: This story has been updated
to correct the amount of time between an earthquake and the
arrival of the Mormons in Utah, which was 247 years, not
147.